FIG. 1 illustrates an aircraft fuselage 3 powered by tail mounted, counterrotating, pusher prop fans 6A and 6F. The prop fans are driven by an engine (not shown) contained within a nacelle 9. Extending between the nacelle 9 and the aircraft fuselage is a pylon 12, more clearly shown in FIG. 2. The pylon is an aerodynamic fairing which surrounds the structure which supports the engine, and other apparatus such as fuel and electrical lines, which connect to the engine.
The pylon 12 sheds a wake 15 during flight, no matter how well the pylon is designed.
One reason is that, as shown in FIG. 2A, the thickness 16 of boundary layer 16A progressively increases in the downstream direction 17, causing a velocity profile 17A to exist at the trailing edge 17B. (The velocity profile 17A illustrates the variation in velocity of air molecules as a function of distance from the pylon. For example, velocity vector 17C represents the air velocity at distance 17D from the pylon centerline 17E.)
The velocity profile 17A at the pylon trailing edge produces a "velocity defect," V.sub.d in the wake which is the difference in velocity between the freestream velocity, V.sub.o (outside the wake), and the local velocity, V.sub.1, in this example.
The velocity defect region has an accompanying "mass flow defect," and consequently, air, indicated by path 17H, tends to be entrained into the velocity defect region 17F causing turbulence.
A second reason is that the angle of attack of the fuselage 3 will change during flight, while the pylon is designed for producing a minimal wake at a single, optional angle of attack. Consequently, the pylon produces a larger wake at angles which differ from the optimal angle.
The wake has two undesirable side effects. One, when the prop fan blades pass through the wake 15, they chop the wake, producing noise. An exaggerated example, shown in FIG. 3, will illustrate this point. When a ship's propeller 18 operates partly submerged in water 21, each blade makes a noise as it enters the water. The water can be viewed as analogous to the wake 15 in FIG. 2; noise is produced when each propeller blade passes through the wake 15.
If each prop fan has eight blades and rotates at 20 revolutions per second, then 160 chops occur per second. This situation resembles a noise source broadcasting at 160 Hz, together with overtones.
A second side effect results from the fact that the lift produced by a propeller blade is a function of the angle of attack of the blade with respect to the incoming air. When a blade enters the wake 15, the angle of attack changes as shown in FIG. 2B.
The angle of attack A1 experienced by blade 6A is the vector "sum 1" of two vectors: (1) freestream vector V.sub.o, resulting from the forward speed of the aircraft and (2) vector "rotation" representing the rotational velocity of the blade 6A.
When the blade 6A enters the wake, freestream vector V.sub.o becomes reduced, as illustrated by vector V.sub.1 (also shown in FIG. 2A). Consequently, the vector "sum 1" changes to vector "sum 2." This latter vector "sum 2" causes a higher angle of attack A2 to occur.
As a result, the prop fan blade 6A becomes more highly loaded, and the lift load in the direction of arrow 26 of FIG. 1 (thrust direction) becomes greater, causing the blade to flex in that direction. Given that, for example, the type of engine shown in FIG. 2 can be of the 25,000 pound thrust class, and that a total of sixteen prop fan blades can be used, the total loading per blade is approximately 1,560 pounds (25,000 divided by 16). Even a small percentage increase in thrust during passage through the wake 15, such as a ten percent increase, can cause significant stresses on the blades and potential damage to the blades over a period of time. An example will illustrate this.
Assume that the blade radius 31 in FIG. 2 is five feet. Thus, the circle described by the tip 33 of each blade is approximately thirty-one feet in circumference. (Five-times-two-times-pi equals approximately thirty-one.) Assume, as above, a speed for each prop fan of twenty revolutions per second. Therefore, in this example, the tip region 33 of each blade is traveling along the circumference at the rate of about 620 feet per second. (Thirty-one feet per revolution times twenty revolutions per second.)
If the wake is assumed to be one-foot high, dimension 38, and if it is assumed that the wake provides a ten percent increase in lift during an excursion by a blade through it, then each blade experiences an impulse of ten percent of the thrust load, or about one-hundred-fifty pounds, applied during an interval of 1/620 seconds, that is, an interval of about 1.6 milliseconds. Further, at 20 revolutions per second, each blade passes through the wake once every 1/20 second, or every 50 milliseconds. Restated, a cyclic load of 150 pounds is applied for 1.6 milliseconds to each blade every 50 milliseconds. It is clear that such a cyclic loading should be avoided.